<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>红绿灯倒计时</title>
    <style>
        .box {
            width: 250px;
            height: 52px;
            padding: 15px 30px;
            border: 2px solid #ccc;
            border-radius: 16px;
            margin: 0 auto;
        }
        
        .box .count {
            width: 60px;
            color: #666;
            font-size: 280%;
            line-height: 50px;
            padding-left: 6px;
            margin-left: 5px;
            border: 1px solid #fff
        }
        
        .box div {
            margin-left: 5px;
            float: left;
            width: 50px;
            height: 50px;
            border-radius: 50px;
            border: 1px solid #666;
        }
        
        .gray {
            background-color: #eee;
        }
        
        .red {
            background-color: red;
        }
        
        .yellow {
            background-color: yellow;
        }
        
        .green {
            background-color: #26ff00;
        }
    </style>

</head>

<body>
    <div class="box">
        <div id="red"></div>
        <div id="yellow"></div>
        <div id="green"></div>
        <div class="count" id="count"></div>
    </div>
    <script>
        var lamp = {
            red: {
                obj: document.getElementById('red'),
                timeout: 30,
                style: ['red', 'gray', 'gray'],
                next: 'green'
            },
            yellow: {
                obj: document.getElementById('yellow'),
                timeout: 5,
                style: ['gray', 'yellow', 'gray'],
                next: 'red'
            },
            green: {
                obj: document.getElementById('green'),
                timeout: 35,
                style: ['gray', 'gray', 'green'],
                next: 'yellow'
            },
            changeStyle(style) {
                this.red.obj.className = style[0];
                this.yellow.obj.className = style[1];
                this.green.obj.className = style[2];
            }
        };
        var count = {
            obj: document.getElementById('count'),
            change: function(num) {
                this.obj.innerHTML = (num < 10) ? ('0' + num) : num;
            }
        };
        var now = lamp.green;
        var timeout = now.timeout;
        lamp.changeStyle(now.style);
        count.change(timeout);
        setInterval(function() {
            if (--timeout <= 0) {
                now = lamp[now.next];
                timeout = now.timeout;
                lamp.changeStyle(now.style);
            }
            count.change(timeout);
        }, 1000);
    </script>


</body>

</html>